Developmental specialization of the left intraparietal sulcus for symbolic ordinal processing.

Symbolic numbers have both cardinal (symbol-quantity) and ordinal (symbol-symbol) referents. Despite behavioural evidence suggesting distinct processing of cardinal and ordinal referents, little consensus has emerged from the neuroimaging literature on whether these processes have shared or distinct neural underpinnings. Moreover, it remains unclear how the neural correlates of cardinal and ordinal processing change with age. To address these unresolved questions, we investigated the neural correlates of cardinal (neural distance effect) and ordinal processing (neural reverse distance effect) in 50 children (ages 7-10) and 26 adults (ages 19-26). We found that adults recruited a largely left lateralized set of fronto-parietal regions for ordinal processing, whereas children showed activation in the right lateral orbital and inferior frontal gyri for both ordinal and cardinal processing. Additional analyses suggested that adults recruited the left intraparietal sulcus (IPS) more than children for ordinal processing, suggesting that the IPS may become increasingly tuned to ordinal symbolic properties over development. Together with previous literature documenting the importance of the left IPS for cardinal processing, our results suggest that cardinal and ordinal processing may share neural substrates in the left IPS and that this region may become specialized for both skills over development.